A VHF tuner having a conventional diode mixer circuit 11 is shown in FIG. 1, wherein a crystal oscillator circuit 12 feeds a multiplier diode 14 load from the emitter of transistor 16. By driving the multiplier diode 14 heavily in the forward direction, the non-linear properties of the diode cause heavy harmonic generation. A parallel resonant circuit 18, comprised of inductor 19 and capacitor 21 which are tuned to the desired harmonic, causes a chosen harmonic to be injected into the input of the mixer transistor 20 through a coupling capacitor 22. Such multiplier diode harmonic generators constitute inherently low impedance sources, as seen from the parallel resonant circuit 18, with the result that the Q of the resonant system is quite low, typically resulting in poor selectivity characteristics with respect to the chosen harmonic to be injected into the mixer transistor 20. Thus, for example, harmonics well below or well above the desired injection frequencies will also be present at the input of mixer transistor 20 and can result in image reception from unwanted stations. Accordingly, it is desirable to inject to the input of the mixer 20 a specifically selected harmonic from the crystal oscillator circuit 12, without the presence of other such undesired harmonics.
In a VHF tuner circuit, an improved circuit for injection of a desired harmonic of the oscillator frequency into the mixer stage and suppressing all other harmonics.